Add support for importing and exporting wisdom files

[SallySoul]

First, I would be interested in submitting a PR for this, but I wanted to get feedback first.

For reference, FFTW3 can save and later load accumulated wisdom about optimal plans it has found.

The symbols I'd want to add are a subset of import and export. In particular importing and exporting to a filename.

The fftw-sys crate already exports these symbols, so it would be a matter of adding them to the fftw crate proper. Perhaps to a util or wisdom module?